1. Field of the Invention
The present invention pertains to orthodontic appliances for correcting teeth malalignment. Specifically, the present invention pertains to twin orthodontic brackets of the edgewise type.
2. Brief Description of the Prior Art
For many years the most popular, widely used, effective and mechanically sound orthodontic appliances used in correcting imperfections in the alignment of the teeth have been those appliances generally referred to as the "edgewise" type. Although there are several variations of the edgewise appliance, it normally includes brackets which are attached to each tooth in some fashion and at least one arch wire which spans the teeth engaging a slot in each of the brackets. The arch wire may be affixed to each bracket by connecting wire, resilient band or the like. A typical edgewise bracket is sown in U.S. Pat. No. 3,250,003.
Most edgewise brackets include a base portion affixed to a pad which is adapted for attachment, in any suitable manner, to the outer surface of a tooth. Upper and lower wing portions project outwardly from the base portion defining therebetween an outwardly opening slot or groove for receiving the arch wire. The wing portions provide a means of tying or holding the arch wire in place within the slot. The height of the bracket slot (distance between wing portions) is precisely formed to receive the normal rectangular cross-section arch wire in a very precise or tight fit.
For many years, the height of the slot in edgewise brackets was designed for receiving an arch wire, the edgewise dimension of which was 0.022 inches. Such a wire is relatively stiff and exerts an often unnecessary amount of force on the teeth. Consequently, many orthodontists began using edgewise appliances with slots designed to receive arch wires having an edgewise dimension of 0.018 inches. Through the years, it has become apparent that the larger dimensioned brackets, when used with small wires, had several advantages: greater range and resiliency of the wires used, and less permanent deformation of the arch wires due to occlusive forces during mastication. However, the loose fit between a larger dimension bracket and a small wire results in reduced control of the teeth in certain areas.
U.S. Pat. No. 4,793,804 discloses a bracket having a stepped slot which, in effect, provides a small height slot near the base and a larger height slot at the outer opening thereof. Thus, the stepped slot will accommodate, in the portion of the slot nearest its base, a smaller arch wire in a precision manner, or in the portion of the slot nearest its opening, in a loose fit. For example, an orthodonist may use a small wire with a larger portion of the slot during initial treatment. For intermediate treatment, he may use a small wire with large slots (loose fit) on some teeth and small slots (tight or precision fit) on other teeth. In finishing, a small wire may be used with a small slot on all teeth.
Single brackets of the type shown in the previously mentioned U.S. Pat. Nos. 3,250,003 and 4,793,804 provide maximum effectiveness in the application of tipping and torquing of teeth but are not efficient in rotational control. Thus a number of other types of brackets or appliances have been developed to improve over the inefficient rotation of the typical bracket. One of the most accepted types is the twin bracket which instead of one centrally located bracket, provides a pair of spaced apart brackets on a single tooth. When each of these pairs of brackets is tied to the arch wire, there is a tendency of the tooth to rotate so that the facial surface of the tooth aligns itself with the arch wire.
Twin brackets, which have been used for a considerable period of time, typically incorporate a pair of spaced apart brackets each of which defines a precision arch wire slot. The spaced arch wire slots cooperate to provide the effect of a precision arch wire slot extending the entire length of the bracket base. Each of the brackets is provided with upper and lower tie wings, defining a bracket structure with four tie wings. The tie wings in the effective arch wire slot terminate at the opposed sides of the tooth. With the twin bracket centered in respect to the tooth to be moved, the tie wings are thereby positioned in pairs at opposed sides of the tooth to provide a bracket structure with efficient rotation control.
While twin brackets of the prior art offer substantial advantages in rotational control, they are not without disadvantage. They may require frequent patient visits for adjustment to maintain the necessary force levels for efficient tooth movement. Because of the minimum interbracket width provided the arch wire is not allowed to flex as much as with single brackets. This may cause undue discomfort to the patient. In addition, the typical prior art twin bracket may not permit certain control measures for correcting other tooth misalignment problems. For these and other reasons various modifications have been made in the twin bracket. For example, U.S. Pat. No. 4,531,911 discloses a variation of the twin orthodontic bracket which is claimed to overcome some of these disadvantages.